Of course, I also notice that my focus changes a bit, but it still stays on the traffic around me and I would consider it rather improbable that I would oversee a stopped vehicle infront of me, not to mention a fire truck on a mission.
Maybe it's also that I'm driving daily in and out of Stuttgart, one of the main traffic spots in southern Germany. There's a lot of cars and ignorant driving, especially in dense traffic or jams. Plus we have a rediculous amount of roundabouts in the meantime and there are a lot of people that decide to either push into the smalles gaps, shooting in from your side and forcing you to do heavy breaking or other people approaching an empty roundabout, doing a needles near-full stop, accelerate, just to decellerate again, almost coming to another near-full stop to get around the corner (never understand the thinking behind this :-| ). I don't trust adaptive cruise control in either of these situations.
Maybe you're not a typical case?
FWIW, I consider myself a pretty decent focussed driver, yet when I hired a car with dynamic cruise control a few months back, I was shocked how much losing responsibility for part of the driving experience --the gears, acceleration, and braking, obvs-- helped me to relax too much, and lose a little bit of focus. I was still steering, and it wasn't like there were even any near misses... but I just felt my attention wander too many times for comfort.